Chapter 2: Taking a closer look
Unpacking diversity: How to read a evolutionary tree
- Depicts evolutionary relationships amongst a selected taxa
- hypothetical
- taxon (singular) can be species or groups
- Taxa at ends of tree are usually living (extant)
- taxa not at the end are extinct
- Taxa are connected by branches, which join at nodes
- node represents ancestor shared by 2+ taxa
- branch length may or may not mean something
- Infer degree of relationship based on nodes
- 2 human virus are more closely related to one another that either is to bat virus.
Nodes in cladograms represent divergences or speciation events
Each node indicates the common ancestors of descendants
Unique history (divergence or speciation) based represents key molecular change
Often represented as morphological trait in a fossil-based phylogeny
The molecular story of life
Change is the only constant
Evolution keep rolling, so species are not fixed in a hierarchy
Evolution keep rolling, so species are not fixed in a hierarchy
Change occurs across biological scales
- Genes - Organisms - Species - Populations - Communities - Ecosystem - Biosphere
- Are are interconnected and thus all are influenced by environmental change
Change occurs at molecular level
- Once there is variation, there are two main processes that can work on that variation and lead to long term evolution
- Natural Selection
- Genetic Drift
Natural selection in the wild
Natural selection in the wild
White Sands formed recently (2,000 years old), and the substrate color of the white gypsum habitat contrasts dramatically with the surrounding Chihuahuan Desert’s dark soils habitat
The adaptive value of cryptic color
A very dramatic selection environment leading to concealing coloration for predation avoidance
Multiple colonizing lineages
Genetic drift
More gametes are produced than seed the next generation
Genetic drift
Random sampling gametes can change the genetic composition of a population over time
Genetic drift can have rapid effects during a “bottleneck
Genetic drift in the wild
Selection and Drift
- Selection and drift are both always happening simultaneously
- drift is by chance alone
- selection is by reproductive success
- But they are not always equally strong
- Both processes can lead not only to change within populations but also divergence between populations
Selection and Speciation
